Total hip arthroplasty (THA) (also called surgical replacement of the hip joint with an artificial prosthesis (total hip replacement)) is a reconstructive procedure in which the damaged bone and cartilage are removed and replaced with prosthetic components. Total hip arthroplasty is an often chosen treatment option for people with late-stage degenerative hip disease; however, chronic pain and impairment of daily function of patients with severe hip arthritis are also reasons for considering treatment with total hip replacement.
In THA, one of the components that is needed to reconstruct the hip is an acetabular cup which is used to replace the natural socket of the patient which is called the acetabulum. The acetabular cup is the part of the hip implant that forms the socket in the ball-and-socket structure of the hip joint. The femoral head at the top of the femur rotates within the curved surface of the acetabulum. Accordingly, the THA procedure involves replacing the acetabulum (socket) with the acetabular cup and a femoral implant that includes a femoral head (ball) and a stem that attaches to the femur bone.
Implant positioning is of critical importance in primary total hip arthroplasty. Acetabular cup position is traditionally described by its centre of rotation (acetabular component offset), its anteversion (CV), and its inclination (CI, also termed abduction). Incorrect acetabular cup placement is associated with higher dislocation rates, range of motion limitations due to impingement, eccentric polyethylene wear, and ultimately, higher rates of revision.
Conventional techniques used to determine acetabular cup position include external alignment guides, free-hand positioning and the use of anatomic landmarks. Previous studies demonstrated that these techniques allow for correct positioning of the acetabular component in the target zone in only 50-86% of the cases. In order to avoid implant malposition, a variety of imageless and image-based navigation techniques have been developed.
More specifically, perioperative imageless techniques are primarily based on infrared optical stereoscopy and involve an optical localizer capturing the position of an optical tracker, which is fixed to the patient thus allowing for the three-dimensional tracking of tools and prosthetic components. These methods do not expose the patient to radiation nor do they require a specific patient position, but they necessitate costly dedicated hardware and perioperative registration and calibration procedures that are time consuming. In combination with surface mapping based on Computer Tomography (CT) images this technology is also used in robotic assisted surgery.
As mentioned herein, one surgical technique for THA is referred to as the direct anterior approach (DAA). In general, direct anterior hip replacement is a minimally invasive surgical technique which involves a 3 to 4 inch incision on the front of the hip that allows the joint to be replaced by moving muscles aside along their natural tissue planes, without detaching any tendons. The introduction of the direct anterior approach (DAA) with the patient lying in a supine position has greatly facilitated the use of intraoperative fluoroscopy. As is known, fluoroscopy is a type of medical imaging that shows a continuous X-ray image on a display (monitor), much like an X-ray movie. During a fluoroscopy procedure, an X-ray beam is passed through the body.
Fluoroscopic control during THA provides standardized anterior posterior (AP) images of the hip, which can improve acetabular cup placement. However, the assessment of the CV (anteversion) angle has proven to be more complicated. Image-based techniques relying on standardized AP pelvic films such as those introduced by Lewinnek and Liaw have been validated but are not easily available intraoperatively.
There is therefore a need for an improved method for determining acetabular anteversion using intraoperative fluoroscopy for patients undergoing anterior or anterolateral total hip replacement in a supine position.